Sheets of corrugated board, paperboard, fiberboard or other sheet material are conventionally conveyed to a stacking hopper on an entry or line conveyor. In some cases, the sheets are overlapped or shingled, while in other cases, gaps in the direction of movement are provided between adjacent sheets. Overlapping or shingling of sheets is often undesirable. For example, because shingling results in conveyance of a solid stream of sheets, sensor identification of the location of individual sheets and the presence of jams or misalignments along the conveying path can be difficult. Moreover, the shingling of sheets results in a higher sheet density along the conveyor (i.e., number of sheets per unit area of conveyor), which may result in an increase in the occurrence of jams as well as increase in the number of sheets involved in the jams. Still further, because many of the sheets have flaps or other protrusions at their leading edges, shingling of sheets can be problematic.
Typically, the sheets are projected off the end of the entry conveyor and over a stacking hopper. The stacking hopper includes a generally vertical backstop and a forwardly positioned back tamper to define a bin or area to receive the sheets in stacked form. The capacity of a particular sheet stacking apparatus is determined by the number of sheets that can be stacked per unit of time. In general, this is directly related to the speed of the entry conveyor. The greater the speed of the entry conveyor, the greater the number of sheets that can be stacked in a unit of time, and thus the greater the stacking capacity of the sheet stacking apparatus. As the speed of the entry conveyor is increased, however, the sheets are projected over the stacking hopper and against the backstop at an increased speed. At elevated speeds beyond a certain speed (usually about 300 feet per minute for certain sheets), the projection against the backstop results in the sheet bouncing back toward the entry conveyor and/or possible damage to protruding tabs or flaps on the leading edge of the sheet. Accordingly, without deceleration means, a sheet stacker has a certain maximum effective operational speed.
To improve the capacity of the stacker beyond that point, it is necessary to decelerate or slow the speed of the sheets as they leave the entry conveyor and before they reach the backstop. The prior art includes various deceleration apparatus that function to decelerate or slow the speed of the sheets in this region. One such prior art machine utilizes a set or pair of spatially fixed nip rollers at the end of the entry conveyor and prior to the stacking hopper. In this particular apparatus, the nip rollers are positioned on opposite sides of the sheet and are designed to run or be driven at the entry conveyor line speed for most of the length of the sheet. As the trailing edge of the sheet approaches these rollers, they are decelerated to a desired lower speed to slow the sheet. After the sheet has passed, the rollers are accelerated back to line speed before the next sheet arrives. A limitation of this apparatus includes the physical limitations of ramping the rollers up to about 1,000 feet per minute or more and then back down to about 500 feet per minute or less at least three times per second. A further limitation or disadvantage includes machine wear and tear associated with this repeated high speed acceleration and deceleration.
A further deceleration apparatus, such as that disclosed in U.S. Pat. No. 7,052,009, titled “Sheet Deceleration Apparatus and Method,” issued May 30, 2006, and incorporated by reference herein in its entirety, utilizes a pair of rollers moveable toward and away from one another to nip the sheet traveling between them. Specifically, this method involves delivering a sheet between the pair of rollers and moving the rollers toward one another to nip, and thus decelerate, the sheet as it enters the area of the stacking hopper.
Yet another deceleration apparatus utilizes an overhead vacuum to transport the sheet into the hopper area. This machine ramps the speed of the vacuum conveyors down to zero, kicks off the end sheet over the hopper, and then ramps back up to line speed. Although this machine is acceptable at lower speeds, it is expected that it would have drive problems at higher speeds. A combination of the deceleration apparatus of U.S. Pat. No. 7,052,009 and various embodiments of overhead vacuum means is further described in U.S. patent application Ser. No. 12/351,496, titled “Sheet Deceleration Apparatus and Method,” filed Jan. 9, 2009, which is incorporated by reference herein in its entirety.
Accordingly, there is a continuing need in the art for a sheet deceleration apparatus and method which overcomes the limitations in the art and provides a deceleration method and apparatus capable of increasing the stacking capacity of a sheet stacker. Additionally, there is a continuing need in the art for a sheet deceleration apparatus and method that can lower complexity and/or part count, increase reliability, lower power requirement, and/or allow faster conveyor line speeds.